I've been away from this forum a while. But I will answer this old question just in case someone looks it up.
There were 2 Q6600. One was 105W the newer G0 stepping was 95W. It will have SLACR marked on it.
This is probably what you have.
With a piece of tape it will go 3.0GHz. Look up Q6600 BSEL tapemod.
That can handle 8GB DDR2 800. But it must be X64 low density type.
The 305W PSU has 2x 18A. 12V. rails. That's over 400W 12V. It just doesn't have the connectors to use it all.
The 375W PSU from a Dell T3400 has a 6 pin GPU cable. Same 2x 18A 12V.. rails. It fits.
The Zotac GTX1050 2GB Mini fits. So does the GTX1050TI 4GB mini. These are 2 slot wide cards with a 1 slot bracket.
They don't need the GPU cable so keep your 305/400W PSU.
Recent games require SSE4. The Q9650, Q9550, and E8600 have this. The 3.33GHz E8600 will often be the best choice.
The Tapemod also works on the E7500 to 3.67GHz but no SSE4.